1. Field of the Invention
The present invention relates generally to a liquid crystal display used for a personal computer or the like, and more particularly, to a frame structure suitable for realization of a thin and lightweight liquid crystal display and the structure of a back light unit in the liquid crystal display.
2. Description of the Background Art
A frame constituting a liquid crystal panel unit for use in a liquid crystal display has been conventionally used for holding a liquid crystal panel and a back light unit. The back light unit is constituted by a plurality of components which are represented by a lens sheet, a photoconductive plate, a diffusion sheet, a reflection sheet, and an electric-discharge lamp. Therefore, it is necessary to fix the back light unit separately using a holding component in order to complete the liquid crystal display.
FIGS. 17 and 18 illustrate respectively examples of frame structures for a liquid crystal display which have been conventionally employed. In the frame structure shown in FIG. 17, a photoconductive plate 102 is only put on a frame 101. Therefore, the photoconductive plate 102 is fixed to the frame 101 using a fastener 103 in order to complete the liquid crystal display.
In the frame structure shown in FIG. 18, a frame 104 is held in a sheathing plate 107, only to hold a photoconductive plate 105 and a liquid crystal panel 106. Therefore, a liquid crystal unit 109 is fixed to a cabinet of the main body of the liquid crystal display by a screw hole 108 provided in the sheathing plate 107. The above-mentioned conventional liquid crystal display is thus constituted by a frame for supporting each of components constituting the back light unit, a fastening component for fastening the frame and the components constituting the back light unit, and a sheathing plate for holding the frame. Therefore, the number of components is large, so that the liquid crystal display is thick and is heavy.
At the same time, the liquid crystal display is widely employed for applications such as a portable notebook-sized personal computer and a portable liquid crystal TV. Such a portable liquid crystal display is driven by a battery, so that power consumed by the liquid crystal display prevents battery life from being lengthened. Power consumed by a back light used for the liquid crystal display is very large in the consumed power. The optical efficiency of the back light is improved, to increase the luminance of a display screen by small consumed power, thereby lengthening the battery life.
FIG. 19 illustrates the general construction of such a back light. In FIG. 19, reference numeral 50 denotes a light source such as an electric-discharge lamp, which is arranged opposite to a side surface of a photoconductive plate 51. In order to effectively introduce light into the photoconductive plate 51 from the light source 50, the rear surface of the light source 50 is covered with a reflection sheet 52. A reflection sheet 53 is closely superimposed on the lower surface of the photoconductive plate 51, and a diffusion plate 54 and lens sheets 55 and 56 are closely superimposed on the upper surface thereof, to constitute a back light unit 57. A liquid crystal panel 58 is arranged at some distance above the back light unit 57, to constitute a liquid crystal panel unit.
Light irradiated from the light source 50 is incident on the photoconductive plate 51, and the light travels in the photoconductive plate 51 to expand over the whole surface thereof, and is incident on the lens sheets 55 and 56 through the diffusion plate 54. The incident light is refracted in the lens sheets 55 and 56 toward a viewer, to converge in his or her field of vision at an angle of less than approximately 70 degrees. Light which reaches the viewer is doubled by inserting the lens sheets 55 and 56. Each of the lens sheets 55 and 56 is constructed by arranging very small prisms on a transparent film. The lens sheets 55 and 56 are made different in the angles of view in the X and Y directions by making the directions in which the prisms are arranged perpendicular to each other. In FIG. 19, an array of prisms in the lens sheet 56 is drawn in a vertical direction, and the lens sheet 55 is drawn in a horizontal direction. The optical efficiency of the back light is improved by arranging the lens sheets 55 and 56 thus constructed in a correct direction and in a correct order on the diffusion plate 54, thereby making it possible to increase the luminance of the display screen of the liquid crystal display by small consumed power.
FIG. 20 illustrates an example of positioning of lens sheets in a conventional back light unit. In FIG. 20, reference numeral 59 denotes a supporting member for supporting lens sheets 55 and 56, and reference numeral 60 denotes a notch provided at corners of the lens sheets 55 and 56. The notch 60 prevents one of the surface and the reverse surface of each of the lens sheets 55 and 56 from being mistaken for the other in assembling the lens sheets.
In this method, however, it is difficult to find out, even if in inherently inserting two lens sheets, insertion of one of them is forgotten, that the insertion has been forgotten. It is impossible to confirm the order in which the lens sheets 55 and 56 are inserted even if they are both inserted. Further, the diffusion plate 54, the photoconductive plate 51, and the reflection sheet 53 are inserted with them being superimposed on the lens sheet 56, and thus it cannot be physically confirmed whether or not the components constituting the back light unit 57 are correctly mounted after completion of the back light unit 57.
Therefore, a unit has been conventionally inspected by mounting the liquid crystal panel 58 and operating the display screen to observe the luminance, the viewing angle, and so forth. As a result of the inspection, there are only measures to drop the component constituting the back light unit 57, whose operation is judged to be abnormal, from an assembly line, and decompose and repair the liquid crystal panel 58 and the back light unit 57.
Therefore, the present invention makes, as a personal computer is made small in size, thin, and lightweight, a liquid crystal display carried thereon small in size, thin, and lightweight. Further, an object of the present invention is to provide a back light unit, in the liquid crystal display, capable of preventing one of the surface and the reverse surface of each of lens sheets to be inserted from being mistaken for the other and preventing an erroneous number of lens sheets from being inserted in assembly processes of the back light unit as well as visually confirming the number of lens sheets to be inserted and the order in which the lens sheets are inserted even after assembling the back light unit.